As a device for measuring an inner diameter of a hollow member or a hollow shaft, a device disclosed in the Patent Document 1 is known, for instance.
In the Patent Document 1, an inner diameter measuring device is disclosed, which comprises a main body supporting unit, being able to run through interior of a pipe, a main body unit supported on the main body supporting unit, and an inner diameter measuring unit installed on the main body unit.
The inner diameter measuring device disclosed in the Patent Document 1 is to measure interior portion of a pipe, i.e. to measure inner surface by running along an inner surface of the pipe. In a case where there is a step or surface irregularities on the inner surface, the inner diameter measuring device may not be able to run, or a decentering may occur on the inner diameter measuring unit due to shock caused when the inner diameter measuring unit moves over the step or surface irregularities, thereby causing problems such as decrease of measurement accuracy.
Also, in the Patent Document 2, an optical inspecting device for inspecting inner wall of a tubular product is disclosed, which comprises a tubular casing supported at the center of a pipe by an aligning device used as a guide to inspect inner surface of a pipe, an optical system installed inside the tubular casing and used for projecting laser beams over total circumference, and an electronic camera for taking an image of an optical ring, which is formed when the laser beams are projected to an inner surface of the pipe, and performing image processing of the optical ring photographed by the electronic camera.
The optical inspecting device of the Patent Document 2 also performs aligning by using an inner surface of the pipe as a guide and moves along the inner surface. There have been problems in that the device cannot run where there is a step or surface irregularities, or decentering may occur and measurement accuracy is decreased.
To solve the problems as described above, it is an object of the present invention to provide an inner diameter measuring device, which can support the inner diameter measuring unit without being brought into contact with inner surface of the hollow portion, being able to perform inner diameter measurement correctly regardless of a shape or a property of the inner surface of the pipe and, moreover, able to perform inner diameter measurement even in case an object to be measured has longer length.